| Die-upset is an effective method to produce bulk anisotropic Nd-Fe-B magnets with nanostructure and full density. However, the main drawback of the low coercivity for die-upset Nd-Fe-B magnet limits their applications. For some applications, high coercivity is necessary. As we all know, the coercivity of the permanent magnets are sensitive to their microstructure. In particular, the grain boundary plays a key role in the coercivity. It is an effective way to improve the composition and structure of the grain boundary phase by grain boundary diffusion, which forms a thicker grain boundary phase and helps to obtain high coercivity die-upset magnets.In this paper, anisotropic magnets were prepared by Nd-Fe-B mixed with RE-Cu powder as raw material using hot-pressing and die-upset method. In this way, high coercivity die-upset magnets were obtained successfully, keeping a balance on Br, (BH)m and Hcj. At the same time, the preparation process and RE-Cu composition were optimized. The influence of magnets on microstructure, magnetic property and the cause of coercivity change after RE-Cu alloy addition were discussed. The following is the main content and conclusion of this paper:(1) Through optimized RE-Cu melt-spun process, smooth surface, consistent thickness and homogeneous microstructure ribbons are obtained. The process of hot-pressing and die-upset has been optimized, and the high coercivity magnets are obtained. Nd90Cu10, Nd85Cu15, NdgoCu20 (Wt.%) alloys in different amount are added into die-upset Nd-Fe-B magnets, and the coercivity increases at first and then decreases as the amount of addition increase. At the same time, the content of Cu will lead the amplitude of coercivity improvement. The best performance of magnet is as follows:Br= 13.53 kGs, Hcj= 16.1 kOe, (BH)m= 44.07 MGOe.(2) It is observed that the grain size is getting smaller and grain boundary phase is getting thicker after Nd-Cu addition. Besides, with Nd-Cu content increases, the degree of orientation declines. Low melting point alloy can weaken the coupling effect to the grains. Moreover, the coercivity mechanism changes from the nucleation mechanism to the coexisting of the nucleation and the pinning. These lead to coercivity increasing.(3) The heat treatment can enhance the coercivity of die-upset magnet with Nd-Cu addition. The reason is that the grain growth and orientation has completed after die-upset process. Therefore, the heat treatment can improve the uniformity of the grain boundary phase.(4) By adding Pr85Cu15 or Nd60Dy20Cu20 alloy, high coercivity of die-upset magnets are obtained. The effect of Pr-Cu alloy addition is similar to the Nd-Cu alloy. Dy adopted magnets have a bad microstructure and poor magnetic property, which needs more research.The coercivity of die-upset magnet of 16.1 kOe is prepared successfully without heavy rare earth elements addition, which enhances more than 70% compared with original Nd-Fe-B magnet. At the same time, the experiment significantly improves the temperature stability of die-upset magnets, which is of great value to production application. |
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